GEOLOGIC WORK IN THE LAKE SUPERIOR IRON DISTRICT 
DURING 1902. 
By C. K. Leith. 
In the early part of the year there was distributed a general paper 
by C. R. Van Ilise on the Iron Ores of the Lake Superior Region, con- 
taining a general account of the geology of the region and of the 
origin of the ores, and accompanied by small scale maps of some of 
the districts. 
There have also been sent to press during the year Monograph 
XLIII, on the Mesabi iron-bearing district of Minnesota, by C. K. 
Leith, and Monograph XLV, on the Vermilion iron-bearing district 
of Minnesota, by J. Morgan Clements. W. S. Bayley has spent the 
year in the preparation of another monograph, on the Menominee 
iron-bearing district of Michigan, which will be published in 1903. 
The work in all of these districts has been directed by C. R. Van 
Hise. 
MESABI DISTRICT. 
The Mesabi monograph (XLIII), by C. K. Leith, is a volume of 301 
pages, accompanied by a general map of the range, covering the area 
between Birch Lake and Grand Rapids on a scale of 1 mile to the 
inch, and by 33 plates and 12 figures, including many detail maps 
and sketches. 
The Mesabi iron ores have developed from the secondary alteration 
of a rock composed largely of green ferrous silicate granules, resem- 
bling glauconite, and so called by Spurr, but here shown not to be 
glauconite and called " greenalite." The greenalite granules are sup- 
posed to develop in much the same way as iron carbonates of other 
parts of the Lake Superior region, described by Van Hise, and to owe 
their occurrence in the form of granules to organic agencies. The 
concentration of the ores has consisted essentially in the oxidation, 
under weathering conditions, of the ferrous iron in the greenalite 
granules and the segregation of the iron and silica. The alteration 
of the greenalite and concentration of the ore has occurred through 
the agency of moving underground waters and the ore deposits have 
been localized in places where the circulation has been vigorous. 
Broad, shallow synclines in the iron formation (Upper Huronian) have 
exerted a primary control of the circulation of the underground waters 
247 
